The present invention, in some embodiments thereof, relates to energy conversion and, more particularly, but not exclusively, to a direct liquid fuel cell system, which utilizes hydrazine or derivatives thereof as fuel, and to applications employing a fuel cell system.
A fuel cell (FC) is an electrochemical device that continuously converts chemical energy directly to electrical energy as long as a fuel (commonly hydrogen, or hydrogen-containing compounds) and an oxidant (commonly oxygen) are supplied. One of the main advantages of fuel cells is their high energy density (typically 4,000-9,000 Wh/kg), which is about 18 times higher than conventional electrochemical power sources (such as, for example, Pb—PbO2; Zn—O2; Zn—Ag; Ni—Cd; Li-ion etc.).
Fuel cells are characterized by high efficiency compared to internal combustion engines. In addition, fuel cells are ecologically friendly and several types can function at temperatures as high as 100° C.
The development of fuel cells is one of the main directions in the field of new power engineering. Several types of fuel cells based on H2/O2, phosphoric acid, molten carbonate, alkaline, proton exchange membrane, direct methanol and solid oxide were developed in the last two decades [Carrette et al., ChemPhysChem. 2000, 1, 162; Springer et al., J. Electrochem Soc. 1991, 8, 2334; Atkinson et al., Nature, 2004, 3, 17; Steele and Heinzel, Nature, 2001, 14, 345]. However, these fuel cells are still far from mass production due to multiple practical limitations.
Some of the obstacles associated with fuel cell development include complex electrode and cell design, catalysts poisoning and mechanical instability, high catalyst cost, low potential and slow oxidation kinetic.
In the last years, research efforts were focused on development of direct liquid fuel cells utilizing hydrogen-containing compounds such as methanol, sodium borohydride, ammonia-borane and hydrazine [see, for example, Yamada et al. J. Pow. Sour. 2003, 122, 132-137; Evans and Kordesch, Science, 1967, 158, 1148-1152; Asazawa, et al., J. Electrochem. Soc. 2009, 156, B509; Yamada et al, J. Pow. Sour. 2003, 115, 236; and Jamda at al. Electrochem. Commun. 2003, 5, 892-896]. Hydrazine (abbreviated herein and in the art as Hz) based direct liquid fuel cells (DLFC) which functionalize at room temperature were developed in the early 60s. Hydrazine is a low cost powerful fuel, and is a hydrogen-rich compound (12.5%), similar to methanol, which contains more hydrogens compared to sodium borohydride (10.6%).
Hydrazine is considered a hazardous compound, both in its pure form (as N2H4) and as a monohydrate (N2H4.H2O). Yet, hydrazine is non-explosive and non-toxic in diluted aqueous solutions. Moreover, several hydrazine salts, such as, for example, N2H4.H2SO4 are reported as prospective anticancer drugs [see, for example, Upton et al. Tren. Pharm. Sci. 2001, 22, 140-146].
The basic sources of hydrazine in nature are unlimited (N2 and H2) and the recycling of hydrazine from its basic elements (N2 and H2) is relatively simple. In addition, hydrazine decomposition results in byproducts, nitrogen (N2) and water (H2O), which are ecologically friendly.
The electrochemical oxidation of hydrazine in a basic solution produces four electrons, nitrogen gas (N2) and water, as presented in Equation 1 hereinbelow:NH2—NH2+4(OH)−→N2+4H2O+4e−  (1)
The standard potential of hydrazine oxidation (Eo) corresponds to −1.21 V, its theoretical specific electrical capacity corresponds to 3.35 KAh/kg and its specific theoretical power (W) corresponds to 4.05 KWh/kg (3,350·1.21).
The electrochemical properties of hydrazine in alkaline solutions were investigated in the last three decades using different metal catalysts such as platinum (Pt), palladium (Pd), Nickel (Ni), cobalt (Co), gold (Au), silver (Ag) and mercury (Hg).
Amongst the tested metals, Co, Ni and especially Pt-group metals (PGM) were found to perform as the best catalysts for the electro-oxidation of hydrazine.
The use of Pt and Pt-group metals in large scale for commercial fuel cells is limited by cost and practical considerations, imposed by easy poisoning of such electrodes. Cobalt catalyst is characterized by poor stability over time, whereby Ni catalysts produce relatively low current density, 10 times lower than that produced in the presence of Co catalysts. Au and Ag catalysts produce high over-potential (EOP) of about 500 mV, compared to Pt catalysts, and Hg catalysts produce EOP higher than 800 mV, compared to Pt catalysts.
Asazawa et al. [Angewandte Chemie International Edition, Vol. 46, issue 42, pages 8024-8027] disclose a platinum-free direct hydrazine-based fuel cell, which uses an anion-exchange polymer electrolyte, and cobalt or nickel electrodes, and which exhibits performance that is comparable to that of hydrogen polymer electrolyte fuel cell (PEFC) and exceeds that of direct methanol fuel cell (DMFC).
Copper (Cu) anode catalysts have also been tested in hydrazine-based half-cells. Asazawa et al. [J. Pow. Sour. 2009, 191, 362] have tested various catalysts for the electro-oxidation of hydrazine and of hydrazine derivatives and have shown that a Cu catalyst is inferior to cobalt, platinum and other catalysts. Asazawa et al. have reported that a Cu catalyst exhibits poor catalytic performance in comparison to Co and Ni, and that a Cu catalyst is characterized by EOCP value which is more positive compared to Pt, and, in addition, it was shown that the Cu electrode loses its activity upon applying a potential above −0.8 V vs. SHE (standard hydrogen electrode)
Asazawa et al. further reported on a Pt-free hydrazine-based fuel cell, which utilizes hydrazone for generating hydrazine in situ via hydrolysis [see, www.greencarcongress.com/2007/09/daihatsu-develo.html].
U.S. Patent Application No. 2008/0145733, also by Asazawa et al., discloses fuel cells operated using hydrazine and other amine and hydrogen containing compounds as fuel and a cobalt-containing catalyst layer.
Ghasem Karim-Nezhad et al. [Electrochimica Acta 54 (2009)5721-5726] disclose copper (hydr)oxide modified copper electrode for electrocatalytic oxidation of hydrazine in alkaline media. The modified electrode showed improved stability to corrosion and an improved electrochemical performance (a negative shift of about 120 mV as compared to a bare copper electrode). The disclosed Cu modified electrode, however, operates at a working potential of +0.2 V, which is not suitable for fuel cell applications (for fuel cell application an anode potential of at least −0.5 V is needed).
Some background art concerning interactions between hydrazine and copper(II) ions includes Zhiliang Jiang et al. [Anal. Chem. 2008, 80, 8681-8687], which report that Cu(II) ions serve as catalysts for homogenous Hz decomposition.
Fuel cell systems operating with hydrazine as a fuel and various oxidants have been taught. Commonly used oxidants include, for example, air (for oxygen supply), nitrous acid, and hydrogen peroxide.
The theoretical standard potential (Eo) of a hydrazine/hydrogen peroxide fuel cell (N2H4/H2O2) corresponds to 2.99 V (1.21+1.78). Accordingly, the specific theoretical power (W) of such a cell corresponds to 12.1 KWh/kg (4,050·2.99). The efficiency of a N2H4/H2O2 direct liquid fuel cell in about two times higher compared to gasoline engines.
U.S. Pat. No. 3,410,729 teaches a fuel cell system operated by supplying hydrazine to a carbon or nickel anode and supplying hydrogen peroxide to a carbon or nickel cathode.
U.S. Pat. No. 3,811,949 discloses a hydrazine-based fuel cell system comprising metal alloys (e.g., amalgams) as catalysts and oxygen as the oxidant. The main disadvantage in this fuel cell is the use of dangerous mercury contain electrode.
Electrochemical hydrazine sensors were also developed in the last decades [see, for example, Abbaspour and Kamyabi; J. Electroanal. Chem. 2005, 576, 73-83; Ozoemena and Nyokong; Talanta, 2005, 162-168; Karim-Nezhad and Jafarloo, Electrochimica Acta; 2009, 54, 5721-5726]. These electrochemical sensors utilize as catalysts noble metals, transition metals, organic and inorganic complexes, oxides, metal phthalocyanides, metal porphyrines, and more.
Several publications have reported that CuSO4//Cu(II) is an effective promoter for homogenous oxidation of hydrazine [see, for example, J. Ward, J. Am. Chem. Soc. 1976, 98, 7; J. Corey. J. Am.; Chem. Soc. 1961, 83, 2957; J. Rempel, Appl. Catalysis A: General, 2004, 263, 27; Z. Jiang. Anal. Chem. 2008, 80, 8681].
Many electrochemical H2O2 sensors were fabricated, based on different electron mediators such as Prussian blue [Arkady et al. Anal. Chem., 1995, 67 (14), pp 2419-2423], ferrocene (FeC) [Mulchandani et al., Anal. Chem. 1995, 67, 94-100] and others [see, for example, A. Shinishiro; Chem. Sens, v.21 sup.B (2005) 61], however, the methodologies utilizing such catalysts produced a relatively low current.
Shukla et al. described the use of Prussian blue (PB) as an inorganic electron-transfer mediator (on carbon black; C/PB and polymer) as a catalyst for H2O2 reduction in a SB/H2O2 fuel cell [Shukla at al., J. Power sources, 2008, 178, 86]. The taught C/PB electrode was associated with a complicated fabrication protocol and a modest current density of about 35 mA/cm2.
Ferrocene is known as a potent electron-transfer mediator [see, for example, Anthony et al., Anal. Chem., 1984, 56, 667-671; Gagne et al., Inorg. Chem. 1980, 19, 2854-2855]. Ferrocene is chemically stable in acid solutions and is characterized by good absorption to carbon materials (via π-π interaction).
Attempts to adapt C/Fc for fuel cell technology have been described [see, for example, U.S. Pat. No. 7,320,842; and K. Gong, Science, 2009, 223, 760]. The described methodologies, however, involved a treatment at a temperature of 700° C., which results in decomposition of the C/Fc catalyst.
Additional art includes Logan B. E. and Regan J. M., Environmental Science & Technology, Sep. 1, 2006, 5172-5180.
PCT Patent Application entitled “A DIRECT LIQUID FUEL CELL HAVING AMMONIA BORANE OR DERIVATIVES THEREOF AS FUEL”, having Attorney's Docket No. 47113, by the present inventors, which is co-filed on the same date as the instant application, teaches fuel cell systems which utilize ammonia borane as fuel, and a non-noble metal catalyst as at least one of the anode catalyst and the cathode catalyst. Such fuel cells, which comprise copper catalysts, including catalysts made of copper nanoparticles, are described. This PCT patent application claims priority from U.S. Provisional Patent Application No. 60/113,611, filed Nov. 12, 2008, and from U.S. Provisional Patent Application No. 61/230,764, filed Aug. 3, 2009, the teachings of which are incorporated by reference as if fully set forth herein.